1. Field of the Invention
The present invention relates to a pattern measurement apparatus and a pattern measurement method using a charged particle beam, and in particular to a pattern measurement apparatus and a pattern measurement method capable of accurate length measurement regardless of the edge width of a pattern.
2. Description of the Related Art
As a method of measuring the line widths of patterns, there is a method such as a one performing measurement using a scanning electron microscope (SEM). In the scanning electron microscope, a sample is irradiated with and scanned by incident electrons within an electron beam scanning range, and secondary-electrons emitted from the sample are acquired through a scintillator. Then, the quantity of the acquired electrons is converted into luminance values and displayed on a display.
When such a scanning electron microscope is used to manage the characteristics of a semiconductor device, an operation is conducted in general to inspect whether or not a pattern is formed to have a line width within a design standard. In order to manage the line width of a pattern, the following procedure is carried out. The procedure includes steps of displaying a predetermined region of a pattern formed on a photomask on a display, aiming an electron beam at a measurement point set within the displayed region and applying it onto the measurement point, and then acquiring a luminance distribution waveform on the basis of secondary electrons reflected from the measurement point. Thereafter, high level portions in the luminance distribution waveform are regarded as pattern edges, and a width between two pattern edges is determined as a line width. Then, a further determination is performed about whether or not this line width falls within a tolerance range. As a result, if the line width falls within the tolerance range, the procedure moves to the next step. On the contrary, if the line width is out of the tolerance range, the procedure returns to the pattern-formation processing step.
As described above, the measurement of line widths of patterns is important in the manufacturing process of semiconductor devices. Thus, various techniques have been proposed to accurately measure the line widths.
In general, a largest position for the gradient of luminance corresponding to the quantity of secondary electrons is defined as an edge position of a pattern. Meanwhile, Japanese Laid-open Patent Publication No. 05-296754 discloses an edge detection method in which a local minimum value position for a secondary electron signal is considered as an edge position.
As described above, in a case where the scanning electron microscope is used to measure the line width of a pattern, there is employed the method in which the largest position for the gradient of luminance is defined as the edge position or the method in which the local minimum value position for the secondary electron signal is defined as the edge position.
However, it has been found that detection of such edge position may result in an inconvenience as follows.
Specifically, as the inclination angles of edge portions of a pattern become steeper and thus a width of each of such edge portions becomes smaller, an accurate length measurement of the pattern becomes impossible. In the pattern length measurement, as described above, the pattern is irradiated with and scanned by an electron beam to create a line profile, and then the line profile is differentiated to calculate the line width. Accordingly, depending on a diameter of the electron beam, it is likely to detect secondary electrons reflecting information including not only the intensity corresponding to the pattern but also the intensity of the beam itself. This results in a situation where the line width cannot be measured accurately.
There has been so far no report regarding a technique to accurately measure a line width from a length measurement value of the pattern with a small edge width.